Andrea J. Dowling

Potentiation and cellular phenotypes of the insecticidal Toxin complexes of Photorhabdus bacteria

The toxin complex (tc) genes of bacteria comprise a large and growing family whose mode of action remains obscure. In the insect pathogen Photorhabdus, tc genes encode high molecular weight insecticidal toxins with oral activity against caterpillar pests. One protein, TcdA, has recently been expressed in transgenic plants and shown to confer insect resistance. These toxins therefore represent alternatives to toxins from Bacillus thuringiensis (Bt) for deployment in transgenic crops. Levels of TcdA expression in transgenic plants were, however, low and the full toxicity associated with the native toxin was not reconstituted. Here we show that increased activity of the toxin TcdA1 requires potentiation by either of two pairs of gene products, TcdB1 and TccC1 or TcdB2 and TccC3. Moreover, these same pairs of proteins can also cross‐potentiate a second toxin, TcaA1B1. To elucidate the likely functional domains present in these large proteins, we expressed fragments of each ‘toxin’ or ‘potentiator’ gene within mammalian cells. Several domains produced abnormal cellular morphologies leading to cell death, while others showed specific phenotypes such as nuclear translocation. Our results prove that the Tc toxins are complex proteins with multiple functional domains. They also show that both toxin genes and their potentiator pairs will need to be expressed to reconstitute full activity in insect‐resistant transgenic plants. Moreover, they suggest that the same potentiator pair will be able to cross‐potentiate more than one toxin in a single plant.

Rapid Virulence Annotation (RVA): Identification of virulence factors using a bacterial genome library and multiple invertebrate hosts

Current sequence databases now contain numerous whole genome sequences of pathogenic bacteria. However, many of the predicted genes lack any functional annotation. We describe an assumption-free approach, Rapid Virulence Annotation (RVA), for the high-throughput parallel screening of genomic libraries against four different taxa: insects, nematodes, amoeba, and mammalian macrophages. These hosts represent different aspects of both the vertebrate and invertebrate immune system. Here, we apply RVA to the emerging human pathogen Photorhabdus asymbiotica using “gain of toxicity” assays of recombinant Escherichia coli clones. We describe a wealth of potential virulence loci and attribute biological function to several putative genomic islands, which may then be further characterized using conventional molecular techniques. The application of RVA to other pathogen genomes promises to ascribe biological function to otherwise uncharacterized virulence genes.

Dissecting the immune response to the entomopathogen Photorhabdus

Bacterial pathogens either hide from or modulate the hosts immune response to ensure their survival. Photorhabdus is a potent insect pathogenic bacterium that uses entomopathogenic nematodes as vectors in a system that represents a useful tool for probing the molecular basis of immunity. During the course of infection, Photorhabdus multiplies rapidly within the insect, producing a range of toxins that inhibit phagocytosis of the invading bacteria and eventually kill the insect host. Photorhabdus bacteria have recently been established as a tool for investigating immune recognition and defense mechanisms in model hosts such as Manduca and Drosophila. Such studies pave the way for investigations of gene interactions between pathogen virulence factors and host immune genes, which ultimately could lead to an understanding of how some Photorhabdus species have made the leap to becoming human pathogens.

Drosophila Embryos as Model Systems for Monitoring Bacterial Infection in Real Time

Drosophila embryos are well studied developmental microcosms that have been used extensively as models for early development and more recently wound repair. Here we extend this work by looking at embryos as model systems for following bacterial infection in real time. We examine the behaviour of injected pathogenic (Photorhabdus asymbiotica) and non-pathogenic (Escherichia coli) bacteria and their interaction with embryonic hemocytes using time-lapse confocal microscopy. We find that embryonic hemocytes both recognise and phagocytose injected wild type, non-pathogenic E. coli in a Dscam independent manner, proving that embryonic hemocytes are phagocytically competent. In contrast, injection of bacterial cells of the insect pathogen Photorhabdus leads to a rapid ‘freezing’ phenotype of the hemocytes associated with significant rearrangement of the actin cytoskeleton. This freezing phenotype can be phenocopied by either injection of the purified insecticidal toxin Makes Caterpillars Floppy 1 (Mcf1) or by recombinant E. coli expressing the mcf1 gene. Mcf1 mediated hemocyte freezing is shibire dependent, suggesting that endocytosis is required for Mcf1 toxicity and can be modulated by dominant negative or constitutively active Rac expression, suggesting early and unexpected effects of Mcf1 on the actin cytoskeleton. Together these data show how Drosophila embryos can be used to track bacterial infection in real time and how mutant analysis can be used to genetically dissect the effects of specific bacterial virulence factors.

The insecticidal toxin Makes caterpillars floppy (Mcf) promotes apoptosis in mammalian cells.

Photorhabdus bacteria produce a number of toxins to kill their insect hosts. The expression of one of these, Makes caterpillars floppy (Mcf), is sufficient to allow Escherichia coli to persist within and kill caterpillars. Mcf causes shedding of the insect midgut epithelium and destructive blebbing of haemocytes suggesting it may trigger apoptosis. To investigate this hypothesis, here we examine the effects of E. coli ‐expressed Mcf on the mammalian cell lines COS‐7, NIH 3T3 and HeLa cells. Cells treated with Mcf show apoptotic nuclear morphology, active caspase‐3, DNA laddering after 6 h, and the presence of cleaved PARP after 16 h. These effects are prevented by the apoptosis inhibitor zVAD‐fmk. Transfection of cells with constructs expressing only the NH 2 ‐terminal 1280 amino acids of Mcf, as a fusion with Myc, also triggered cell destruction. The expressed fusion protein was concentrated into the Golgi apparatus before cell death. These results confirm that the novel insecticidal toxin Mcf induces apoptosis but the precise intracellular pathway remains obscure.

Oral Toxicity of Photorhabdus luminescens W14 Toxin Complexes in Escherichia coli

ABSTRACT Previous attempts to express the toxin complex genes ofPhotorhabdusluminescens W14 inEscherichiacoli have failed to reconstitute their oral toxicity to the model insectManducasexta. Here we show that the combination of three genes, tcdA, tcdB, and tccC, is essential for oral toxicity toM. sexta when expression inE. coli is used. Further, when transcription from native toxin complex gene promoters is used, maximal toxicity in E. coli cultures is associated with the addition of mitomycin C to the growth medium. In contrast, the expression of tcdAB (or the homologoustcaABC operon) with no recombinant tccChomolog in a different P. luminescensstrain, K122, is sufficient to confer oral toxicity on this strain, which is otherwise not orally toxic. We therefore infer thatP. luminescens K122 carries a functionaltccC-like homolog within its own genome, a hypothesis supported by Southern analysis. Recombinant toxins from bothP. luminescens K122 and E.coli were purified as high-molecular-weight particulate preparations. Transmission electron micrograph (TEM) images of these particulate preparations showed that the expression oftcdAB (either with or without tccC) inE. coli produces visible ∼25-nm-long complexes with a head and tail-like substructure. These data are consistent with a model whereby TcdAB constitutes the majority of the complex visible under TEM and TccC either is a toxin itself or is an activator of the complex. The implications for the potential mode of action of the toxin complex genes are discussed.

The insecticidal toxin Makes caterpillars floppy 2 (Mcf2) shows similarity to HrmA, an avirulence protein from a plant pathogen

The Photorhabdus luminescens W14 toxin encoding gene makes caterpillars floppy (mcf) was discovered due to its ability to kill caterpillars when expressed in Escherichia coli. Here we describe a homologue of mcf (renamed as mcf1), termed mcf2, discovered in the same genome. The mcf2 gene predicts another large toxin whose central domain, like Mcf1, also shows limited homology to Clostridium cytotoxin B. However, the N-terminus of Mcf2 shows significant similarity to the type-III secreted effector HrmA from the plant pathogen Pseudomonas syringae and no similarity to the N-terminus of Mcf1. HrmA is a plant avirulence gene whose transient expression in tobacco cells results in cell death. Here we show that E. coli expressing Mcf2 can, like E. coli expressing Mcf1, kill insects. Further, expression of the c-Myc tagged N-terminus of Mcf2, the region showing similarity to HrmA, results in nuclear localisation of the fusion protein and subsequent destruction of transfected mammalian cells. The Mcf1 and Mcf2 toxins therefore belong to a family of high molecular mass toxins, differing at their N-termini, which encode different effector domains.

The gene cortex controls mimicry and crypsis in butterflies and moths

The wing patterns of butterflies and moths (Lepidoptera) are diverse and striking examples of evolutionary diversification by natural selection1,2. Lepidopteran wing colour patterns are a key innovation, consisting of arrays of coloured scales. We still lack a general understanding of how these patterns are controlled and if there is any commonality across the 160,000 moth and 17,000 butterfly species. Here, we identify a gene, cortex, through fine-scale mapping using population genomics and gene expression analyses, which regulates pattern switches in multiple species across the mimetic radiation in Heliconius butterflies. cortex belongs to a fast evolving subfamily of the otherwise highly conserved fizzy family of cell cycle regulators3, suggesting that it most likely regulates pigmentation patterning through regulation of scale cell development. In parallel with findings in the peppered moth (Biston betularia)4, our results suggest that this mechanism is common within Lepidoptera and that cortex has become a major target for natural selection acting on colour and pattern variation in this group of insects.

The Yersinia pseudotuberculosis and Yersinia pestis toxin complex is active against cultured mammalian cells

The toxin complex (Tc) genes were first identified in the insect pathogen Photorhabdus luminescens and encode approximately 1 MDa protein complexes which are toxic to insect pests. Subsequent genome sequencing projects have revealed the presence of tc orthologues in a range of bacterial pathogens known to be associated with insects. Interestingly, members of the mammalian-pathogenic yersiniae have also been shown to encode Tc orthologues. Studies in Yersinia enterocolitica have shown that divergent tc loci either encode insect-active toxins or play a role in colonization of the gut in gastroenteritis models of rats. So far little is known about the activity of the Tc proteins in the other mammalian-pathogenic yersiniae. Here we present work to suggest that Tc proteins in Yersinia pseudotuberculosis and Yersinia pestis are not insecticidal toxins but have evolved for mammalian pathogenicity. We show that Tc is secreted by Y. pseudotuberculosis strain IP32953 during growth in media at 28 degrees C and 37 degrees C. We also demonstrate that oral toxicity of strain IP32953 to Manduca sexta larvae is not due to Tc expression and that lysates of Escherichia coli BL21 expressing the Yersinia Tc proteins are not toxic to Sf9 insect cells but are toxic to cultured mammalian cell lines. Cell lysates of E. coli BL21 expressing the Y. pseudotuberculosis Tc proteins caused actin ruffles, vacuoles and multi-nucleation in cultured human gut cells (Caco-2); similar morphology was observed after application of a lysate of E. coli BL21 expressing the Y. pestis Tc proteins to mouse fibroblast NIH3T3 cells, but not Caco-2 cells. Finally, transient expression of the individual Tc proteins in Caco-2 and NIH3T3 cell lines reproduced the actin and nuclear rearrangement observed with the topical applications. Together these results add weight to the growing hypothesis that the Tc proteins in Y. pseudotuberculosis and Y. pestis have been adapted for mammalian pathogenicity. We further conclude that Tc proteins from Y. pseudotuberculosis and Y. pestis display differential mammalian cell specificity in their toxicity.

Insecticidal Toxins from the Photorhabdus and Xenorhabdus Bacteria

Insect pathogens are an excellent source of novel insecticidal agents with proven toxicity. In particular, bacteria from the genera Photorhabdus and Xenorhabdus are proving to be a genomic goldmine, encoding a multitude of insecticidal toxins. Some are highly specific in their target species, whilst others are more generalist, but all are of potential use in crop protection against insect pests. These astounding bacterial species are also turning out to be equipped to produce a vast range of anti-microbial compounds which could be of use to medical science. This review will cover the current knowledge of the lifecycles of the two genera and the potential role of the toxins in their biology, before a more in depth exploration of some of the best studied toxins and their potential use in agriculture.